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From |
lutunnahar begum <pikitardif@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: How to get marginal effect of quadratic term in tobit model in stata 11.0 |

Date |
Wed, 20 Jul 2011 08:19:44 +1000 |

Thank you very much for your help, Maarten. Best, Lutfunnahar On Tue, Jul 19, 2011 at 6:58 PM, Maarten Buis <maartenlbuis@gmail.com> wrote: > On Tue, Jul 19, 2011 at 1:05 AM, lutunnahar begum wrote: >> I tried the following command: >> >> gen tb2=tb^2 >> tobit ch tb tb2 .........., ll(0) robust >> margins, dydx(*) predict (ystar (0,.)) post >> >> I also tried -dtobit- with -version 8.2- and got the 4 MEs for squared term. >> >> I understand that the true ME should not consider the squared effect >> separately as there is only one variable. But papers using tobit model >> report the marginal effect of squared term. I am confused whether I >> should use the above mentioned command to get this. > > Remember that squared terms are just interaction terms of a variable > with itself. In non-linear models the "marginal effect" of the product > term is meaningless, see Norton, Wang, and Ai (2004). So, the authors > in those papers that included the marginal effects of the interaction > terms made a mistake. There are ways of getting "correct" marginal > effects, but those are not worth the effort as they are likely to > confuse more than help. If you added a quadratic term than your > variable will have a non-linear effect, and the way to show that is > with graphs. > > Hope this helps, > Maarten > > Edward Norton, Hua Wang, and Chunrong Ai (2004) "Computing interaction > effects and standard errors in logit and probit models" The Stata > Journal 4(2):154--167. > <http://www.stata-journal.com/article.html?article=st0063> > > -------------------------- > Maarten L. Buis > Institut fuer Soziologie > Universitaet Tuebingen > Wilhelmstrasse 36 > 72074 Tuebingen > Germany > > > http://www.maartenbuis.nl > -------------------------- > > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: How to get marginal effect of quadratic term in tobit model in stata 11.0***From:*lutunnahar begum <pikitardif@gmail.com>

**Re: st: How to get marginal effect of quadratic term in tobit model in stata 11.0***From:*Maarten Buis <maartenlbuis@gmail.com>

**Re: st: How to get marginal effect of quadratic term in tobit model in stata 11.0***From:*lutunnahar begum <pikitardif@gmail.com>

**Re: st: How to get marginal effect of quadratic term in tobit model in stata 11.0***From:*Maarten Buis <maartenlbuis@gmail.com>

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